The present invention relates to the front-end module of a wireless communication device using a send-receive shared antenna and, more particularly, to a front-end module that can be used in a wireless communication device which is applicable for plural communication systems and which makes a switch between plural send and receive signals by a switching circuit connected with a send-receive shared antenna to reduce harmonics radiated from the antenna during transmission. The invention also relates to a communication terminal using this front-end module.
In a terminal device of a wireless communication device as used in mobile communications such as a cellular phone or mobilephone, the sending and receiving systems share one antenna. In this case, a switching circuit is usually inserted between the antenna and the sending and receiving systems. During transmission, the antenna is switched to the transmiting system. During reception, the antenna is switched to the receiving system.
In the terminal device constructed in this way, when transmission is made, an RF send signal produced by the sending circuit of the transmiting system is propagated to the antenna through the switching circuit Then, the signal is radiated as an electromagnetic signal into the air by the antenna. During reception, an RF signal received by the antenna is supplied to the receiving circuit of the receiving system through the switching circuit.
The switching circuit uses a semiconductor device. Even where the sending circuit does not produce harmonics, if a large power such as more than 30 dBm is passed through the switching circuit, harmonics are produced due to nonlinearity of the switching circuit itself.
A configuration in which a filter (such as a low-pass filter for filtering out harmonics) is placed between the switching circuit and the antenna is conceivable as a method of suppressing the harmonics during transmission. However, this provides a loss to the receiving circuit and hence is disadvantageous. In addition, it is theoretically difficult to cope with plural communication systems. For example, there are the following problems. The second harmonic (1800 MHz) of GSM (900 MHz) is close to the fundamental wave of DCS (1750 MHz). The third harmonic (2400 MHz) of GSM (800 MHz) is close to the fundamental wave of PCS (1950 MHz).
Another method of suppressing harmonics during transmission uses a circuit proposed in Japanese Patent Publication JP-A-2001-86026. FIG. 5 is a conceptual view illustrating the manner-in which harmonics from a switching circuit SW are suppressed by a low-pass filter LPF positioned between the switching circuit SW and a transmiting system as shown in the Japanese publication. In FIG. 5, the switching circuit SW produces harmonics toward the antenna and toward the transmiting system at the same time. That is, harmonics (A) are produced toward the antenna. Harmonics (B) are produced toward the transmiting system. If the harmonics (B) traveling toward the transmiting system are well adjusted in phase and reflected by the low-pass filter LPF, the harmonics (B) cancel the harmonics (A) traveling toward the antenna. Thus, the harmonics radiated from the antenna ANT can be weakened.
The conventional circuit described in connection with FIG. 5 can weaken the harmonics on the antenna side to some extent but cannot completely cancel them out. For example, the harmonics suppression level requirement on mobile phones cannot be satisfied in some cases. It is considered that the cause is that when reflection waves (B′) produced by reflecting harmonics (B) traveling toward the transmiting system by a low-pass filter LPF and harmonics (A) traveling toward the antenna are superimposed, they can be put 180° out of phase but cannot be made equal in amplitude, for the following reasons.
(1) Because of asymmetry of the switching circuit SW, the produced levels of harmonics are asymmetric between the input and output ends of the switching circuit SW.
(2) Because the low-pass filter LPF cannot be made loss less, the reflected waves (B′) are smaller in amplitude than the harmonics (B).
(3) Because the reflected waves (B′) attenuate further due to the loss in the switching circuit SW and become reflected waves (B″), when these waves are superimposed with the harmonics (A) in practice, the amplitude is reduced further.